On one hand, with the development of wireless multimedia services, demands of people for high data rate and user experience are gradually increased and higher requirements are raised to system capacity and coverage of traditional cellular networks. On the other hand, application scenarios such as public security, social network, short-distance data share and local advertisement cause demands of people for Proximity Services (ProSe) such as understanding nearby people or things and communicating therewith to be gradually increased. Traditional cellular networks centered on base stations have obvious limitations in aspects such as high data rate and support of proximity services, and a D2D technology, which represents a new development direction of future communication technologies, emerges at the right moment under the background of these demands. The application of the D2D technology can reduce the burden of the cellular networks, decrease the battery power consumption of the user equipment, improve data rate, improve the robustness of network infrastructure and very well satisfy the requirements on the high data rate and proximity services. Current D2D is also called as ProSe or Sidelink Transmission.
The D2D technology can work on licensed bands or unlicensed bands and allow a plurality of user equipment supporting D2D functions, i.e., D2D User Equipment (D2D UE, also called as ProSe enabled UE) to perform direct discovery/direct communication under a situation that there is network infrastructure or there is no network infrastructure. Application scenarios of D2D mainly include three scenarios. FIG. 1 illustrates a schematic view of related D2D application scenarios. As illustrated in FIG. 1, in scenario 1, user equipment perform data interaction under the coverage of a cellular network and user plane data do not pass through network infrastructure; in scenario 2, UE in areas with weak/no coverage perform relay transmission, user equipment (such as UE4 illustrated in FIG. 1) with poor signal quality is allowed to communicate with a network through other adjacent user equipment (such as UE3) under network coverage, thereby operators are helped to expand the coverage and improve the capacity; and in scenario 3, devices directly communicate with one another, it mainly applied to situations such as earthquakes or emergencies when the cellular network cannot work normally, and for example, control planes and user planes between UE5, UE6 and UE7 perform one-hop or multi-hop data communication without passing through network infrastructure.
The D2D technology usually includes a D2D discovery technology and a D2D communication technology, herein the D2D discovery technology refers to a technology which is used for judging/determining whether user equipment are proximal. For related methods, D2D user equipment in scenario 1 discover each other mainly by sending or receiving discovery signals/information, and in scenario 2 and scenario 3, no corresponding solution is determined, i.e., no relay node selection solution is given.